diminishing returns

any rate of profit, production, benefits, etc., that beyond a certain point fails to increase proportionately with added investment, effort, or skill.

2.

Also called law of diminishing returns. Economics. the fact, often stated as a law or principle, that when any factor of production, as labor, is increased while other factors, as capital and land, are held constant in amount, the output per unit of the variable factor will eventually diminish.